This invention relates to the attenuation of background noise in an acoustical field within which a voice microphone is immersed.
There are many applications in which a voice microphone is exposed to an acoustical field of ambient noise creating a problem in transmitting and reproducing intelligible speech sound. Intelligibility research has shown that speech is masked not only by noise of the same frequency but also noises at frequencies higher and lower than speech frequencies. The classical approach to such problem is to utilize a noise-cancelling microphone wherein the front and rear of the microphone diaphragm is exposed to the external noise field in order to cancel noise by virtue of equal pressures exerted on opposite sides of the diaphragm. Such noise cancelling arrangements for voice microphones, however, require microphone dimensions that are relatively small as compared to the wavelengths of the sound being handled. Because of the dimensional limitations involved in the manufacture of such microphones, all frequencies within the speech range cannot be effectively cancelled by the foregoing solution to the problem.
Presently available noise-cancelling microphones require that the microphone be held very close to the lips of a person from which the speech sound originates. Such closeness requirement arises because the pressure gradient established across the microphone diaphragm would otherwise effect cancellation of the speech signals themselves. One disadvantage of such "close talk" requirement of presently known noise cancelling microphones arises in the attachment of such microphone to a flight helmet or headset, for example, by means of a boom and cable introducing additional equipment weight. Other disadvantages of prior art noise cancelling microphones related to the "close talk" requirement involve the hygiene problem arising from the use of the microphone close to the mouth. The microphone with its mesh and cavity design often harbors and encourages the growth of harmful bacteria to which a person may be exposed because of the "close talk" requirement.
It is therefore an important object of the present invention to provide a noise cancelling microphone not limited to close placement relative to the mouth of a speaker. In accordance with such object, it is therefore an additional object of the present invention to provide a noise cancelling. microphone having greater flexibility insofar as placement and mounting is concerned without introducing the complexities of additional compensating equipment.
Yet another object of the present invention is to provide a noise cancelling system for a voice microphone of a conventional single stage type which is less complex and less massive, and may be readily placed or mounted in different environments such as helmets, oxygen masks, etc.
Still other objects of the present invention are to provide a noise cancelling system for voice microphones made useful for a variety of environments by appropriate programming of a sound enhancing acoustical data processor including narrow band voice encoding algorithms, for performance as a function of frequency and increased attenuation and for use in combination with conventional noise cancelling microphones.